It is conventional to etch glass with an aqueous solution of hydrofluoric acid buffered with ammonium flouride. However, this etching solution cannot be employed to etch a glass layer on an aluminum or other metal substrate, such as employed in wafers in the semiconductor industry because the etchant has a tendency to attack the aluminum once it has etched through the glass via.
In an article by D. S. Herman et al, entitled "Multi-Layer Metalization Process CMOS FET Devices" at page 168 of the Electro-Chemical Society Extended Abstracts from the Spring meeting in Washington, D.C., May, 1971, it is stated that the addition of a diol alcohol to the buffered hydrofluoric acid inhibits the aluminum attack. We have determined that a suitable quantity of diol alcohol (ethylene glycol) for this purpose is on the order of 50% of the etching solution. A typical formulation which has been found to be acceptable was 5 parts of buffered oxide etch (34 parts by weight of 40% aqueous ammonium fluoride, plus 5 parts of hydrofluoric acid (49% aqueous)), 5 parts of ethylene glycol and 3 parts of water. One purpose of the 3 parts extra water is to prevent precipitation of ammonium fluoride or complex double salts with silicon fluoride on the semiconductor wafers.
Since the above glycol-containing etchant solution is nearly saturated, it has a high viscosity which renders it difficult to rinse off and to etch small openings in a photoresist. Also, the mixture tends to corrode the aluminum intermittently. Additionally, ethylene glycol has become in scarce supply and is significantly more expensive due to shortages in the petrochemical industry.